Device for producing an intense and uniform magnetic field within a volume of revolution such as a sphere or ellipsoid



3,493,904 NETIC 2 Sheets-Sheet 1 4 W); PMLW JQCQUQS FGUQVEQU J.FAV'EREAU N'G Feb. 3, 1910 DEVICE FOR PRODUCI AN INTENSE AND UNIFORM MAGFIELD WITHIN A VOLUME 0F REVOLUTION SUCH AS A SPHERE OR ELLIPSOID FiledNov. 28, 1967 3, 1970 J. FAVERE AU 3,493, 0

DEVICE FOR PRODUCING AN INTENSE AND UNIFORM MAGNETIC FIELD WITHIN AVOLUME OF REVOLUTION SUCH AS A SPHERE 0R ELLIPSOID Filed Nov. 28, 1967 2Sheets-Sheet 2 Jac ues Favereau United States Patent Int. (:1. HOlf 7/22US. Cl. 335216 10 Claims ABSTRACT OF THE DISCLOSURE A device for theproduction of an intense and uniform magnetic field which comprisesconcentric inductance and shield windings in the form of a volumeenclosing surface of revolution such as a sphere or ellipsoid, the twowindings being made of superconductor material placed in a cryostat andbeing connected in series so that the inductance and shielding effectsare proportional.

The present invention relates to a device for producing an intense anduniform magnetic field in a bounded volume, by avoiding, through thepresence of a shield winding around the inductor element any scatteringof the field in the surrounding space.

It concerns, more particularly, a device of this type which consists ofsuperconductor elements placed in a very-low-temperature cryostat.

The use of superconductors makes it possible to obtainvery highinductances without the continuous consumption of electric power(resistance null) with a reduced volume of materials to channelize thecurrent, due to the very high density of the current that can beadmitted into superconductors of the niobium-zirconium,niobium-titanium, niobium-tin type, for example.

The present invention has for its object:

(1) A device for the production of an intense and uniform magneticfield, of the type comprising an inductor winding and a shield windingdisposed around the former, these two windings being formed ofsuperconductors placed in a cryostat, characterized by the fact that thetwo windings constitute two concentric volume enclosing convex surfacesof revolution, and by the fact that the two windings are electricallyconnected in series so that the induction and shield functions areproportional.

(2) A device in accordance with paragraph 1, in which an access channelto the magnetic field inside of the induction windings is provided nearthe axis of generation of the enclosed volume.

(3) A device according to one of the preceding paragraphs, in which theenclosed volumes are ellipsoids or spheres.

By way of illustration, the invention is described hereinafter withreference to the annexed drawing, in which:

FIG. 1 is a section along the axis of revolution of a device for theproduction of an intense and uniform magnetic field according to theinvention, which is spherical in form.

FIG. 2 is a plan section of FIG. 1, showing the configuration of theconductors used.

By referring to the drawing, it is possible to see at 1 the outerspherical envelope of the cryostat; at 2, its inner spherical envelope;at 3 and 4, the two cylindrical chan nels with a radius R allowingaccess to the inner spherical zone 5 in which the magnetic field ismaximum, uniform,

3,493,994 Patented Feb. 3, 1970 and oriented in parallel to the axis ofrevolution 6 of the system, along the induction line 10.

The induction winding for producing the magnetic field within thespherical inner zone 5 consists of coaxial circular spirals concentricwith the generation axis 6, appropriately distributed over a sphere 7with a radius R The shield winding also consists of coaxial circularspirals concentric with the axis 6, appropriately distributed over asphere 8 with a radius R concentric to the sphere 7.

The two windings are arranged in series, with the fluxes in the inductorind the shield being magnetically in opposition. The distribution of theampere-turns on the spheres 7 and 8 is similar.

The direction of the fluxes has been shown in FIG. 1 by crosses and dotson the spirals of each winding. The inner volume 9 of the cryostatdefined by the space between the inner and outer spherical envelopes 2,1 contains the liquid helium needed to cool the inductor and shieldwindings. In this figure are not shown the conventional thermaninsulation of the inner and outer surfaces (1, 2, 3, 4) of the cryostat,the intakes and outputs of helium in the liquid and gaseous phases, orthe crossovers for the two terminals of the inductor and shieldwindings, which are in series.

When a device of this type is designed to store a magnetic force whichis to be used elsewhere in a pulsed manner, the transfer of the forcecan be accomplished by the previously-mentioned winding or a secondarywinding, by a transformer effect. If this winding is also asuperconductor, it should be placed in the cryostat. If it is not asuperconductor, it can be placed inside of the surface 2 and outside ofsurface 1. In both cases, however, it should represent a distributionsimilar to that of the inductor and shield windings, in order to avoidthe apparition of a magnetic field in the surrounding medium during thetransfer of force.

In certain applications (storage and transfer of magnetic force, forexample), the cryostat can be reduced to its outer envelope 1, asituation which would result in the elimination of surfaces 2, 3 and 4of FIG. 1.

The spherical surfaces shown are a particular case of a volume enclosingsurface according to the invention, i.e. one generated by rotation of awholly convex line about an axis passed through the ends of the line.This geometrical form, either spherical or ellipsoidal,- presentsseveral advantages in practice and, in particular, it ensures a perfectmagnetic shield, contrary to the case of a cylindrical winding, in whichthere is a scattering of the field along the axis, on each side of theWinding; and it is especially well suited to maintain the conductorsagainst the Laplace forces. Furthermore, the cryostats in which thesuperconductor windings are located, and operating at a temperatureclose to that of liquid helium, are simple in form and present a lowratio of the outer surface to the inner volume; this is advantageous intheir thermal insulation. Finally, in the present case of sinusoidaldistribution of the surface current density in the inductor and shieldwindings, the current is null on the axis of generation 6 (axis of thefield); this makes it possible to provide, in the vicinity of this axis,for an access channel with a radius R to the internal uniform magneticfield, without actually disturbing this field.

In the theoretical case of current layers of negligible thickness, withthis sinusoidal distribution of the surface current density, with theform: a =A sin 0 amp/meter-for the inductor with the radius R a =-A sin6 amp/meter-for the shield, with the radius R where 0 is the angle withrespect to the axis 6 defining the position of a spiral (FIG. 1), and a(1 are the current densities in amp/meter, it is possible to obtain auniform field in the inner sphere, while preventing its scatteringoutside of the external sphere, on the condition that the followingrelation is satisfied:

A1R13:A2R23 In addition, if the relation:

R2 :4R13 Or R1 is satisfied, the uniform internal induction is maximal,the radius R is minimal for a given radius R and the induction has thesame value in the equatorial plane (defined by in part and other of thecoiled inductor (radius R In setting the induction at B=5 Wb/rn. andmagnetic force stored in the entire field of W =1MJ, the radii R =23 cm.and R :36.5 cm.

Taking an external diameter of 80 cm. for the cryostat, a megajoule ofmagnetic force is stored in a volume of the order of the quarter of acubic meter, which corresponds to an apparent density of magnetic forceof The inductor and shield windings, connected in series, are excited bythe same current. They present the same true current density in theconductors, or apparent density, if account is taken of the necessaryseparation between conductors for the electrical insulations and thecirculation of helium.

In noting that the quantity A=a/ sin is constant, it is seen that it ispossible to make a spherical winding with a sinusoidal distribution, bymeans of a series of coils with a constant a thickness (FIG. 2).

The apparent density d is defined by d=A /e.

In reverting to the previous numerical example, with A =8- l0 A/m. forthe inductor winding, and d=4-10 A/cm. it is found that e =2 cm. for R=23 cm.

With the same apparent current density a and A =2-10 A/m. for the shieldwinding, it is found that 2 cm. for R =36.5 cm. (FIG. 2).

The thicknesses e and 2 are small as compared to the radii R and R dueto the heavy flux densities that can be admitted into thesuperconductors; this is not too far apart from the theoretical theoryof current layers of negligible thickness.

These coils of constant thickness and of decreasing radius as theyapproach the axis 6, can be given a constant height (il and I1 i.e. aconstant section as shown in FIG. 2. In this figure, the thicknesses eand 2 have been greatly enlarged in order to show more clearly thegeometrical form proposed for the inductor and shield windings.

It is obvious that the invention is not limited to the example of theembodiment described, nor to the form of implementation shown.

I claim:

1. In a device for producing an intense and uniform magnetic field, aninductor winding of superconductor material having a configurationcorresponding to a volume enclosing surface generated by rotation of awholly convex line about an axis passed through the ends of the line,and a shield winding of superconductor material of the sameconfiguration as said inductor winding, said shield winding beingconcentric with and surrounding said inductor winding, andvsaid inductorand shield windings being connected in series.

2. A device as defined in claim 1 wherein the fluxes produced by saidwindings are magnetically in opposition.

3. A device as defined in claim 1 wherein said windings hav a sphericalconfiguration.

4. A device as defined in claim 1 wherein said windings have anellipsoidal configuration.

5. A device as defined in claim 1 wherein said windings consist ofparallel fiat spiral coils the respective planes of which are normal tosaid axis of generation.

6. A device as defined in claim 5 where adjacent spiral coils of saidwindings are essentially contiguous.

7. A device as defined in claim 5 wherein the conductors forming saidspiral coils are of uniform cross-section.

8. A device as defined in claim 1 and which further includes inner andouter concentric spaced surfaces having a configuration corresponding tothe configuration of said inductor and shield windings, said inductorand shield windings being located in the space between said surfaces,and said space being filled with a cooling medium for saidsuperconductor windings.

9. A device as defined in claim 5 wherein the surface current density inthe coils is substantially sinusoidal and an access channel to themagnetic field is provided close to the axis of generation.

10. A device as defined in claim 8 wherein each of said inductor andshield windings consists of flat contiguous spiral coils having auniform cross section and the respective planes of said coils are normalto said axis of generation.

References Cited UNITED STATES PATENTS 3,098,181 7/1963 Cioffi 335-2163,210,610 10/1965 Fraser 3352l6 XR FOREIGN PATENTS 765,937 8/ 1967Canada.

G. HARRIS, Primary Examiner U.S. Cl. X.R. 335299

